The present invention relates to a method and a system for recovery and treatment of aqueous solutions.
In recovery of gas from subterranean formations, the gas that is produced could possibly contain some water. The amount of water produced will vary from one deposit to another. The major portion of the water that is transported with the gas through a pipeline, however, is produced by the so-called Joule-Thompson effect. This involves the effect that the gas is cooled as it loses pressure on moving through a constriction, such as a valve, or through a pipeline. The result is that water in the gas is condensed out. For this reason it is often advisable and/or necessary to inject a hydrate inhibitor. A simple and readily available hydrate inhibitor is methanol. A better alternative, however, is glycol, as glycol will in addition have a corrosion-inhibiting effect.
The water produced will often contain various metals, particularly alkaline earth metals such as magnesium, calcium, strontium, barium and radium. These metals exhibit a tendency to form deposits in the presence of carbonates and sulfates, or if the temperature of the concentration is raised and/or if the pH is increased. The deposits could appear, for example, on heat exchangers and on the inside of pipelines.
For a gas plant in operation, when glycol injection is employed there will be circulated substantial amounts of glycol, which on return from the pipeline will be in the form of a contaminated solution, typically containing about 60% of glycol, water and an increased content of various salts. To permit the reuse of the glycol, a prerequisite is that it must have a concentration of about 90%, and that impurities such as salts, etc., are reduced or eliminated in order to comply with requirements from the process plant.
Glycol injection has previously been used in only a relatively modest degree. One of the reasons for this is probably the existence of precisely these problems associated with recovery of the glycol in a sufficiently pure state.
Some attempts have been made to solve these problems, but few of these seem to function satisfactorily.
At the plant in Bacton, England, the water is removed from the glycol by a process of evaporation. A filter uses compressed air to force the liquid through a filter cloth, and the solid substances are scraped off mechanically. This is a solution to the task that is not well suited for implementation at a gas field.
In the U.S.A. and Canada there are plants for the recovery of triethylene glycol and amines which operate with batch processes. It is assumed that the extent of salt contamination of triethylene glycol (TEG) is relatively low. The operation of these plants generally involves the evaporation and condensation of all liquid under a vacuum. There does not appear to be any practical method for disposing of the salt residues, which are a mixture of salt crystals and other impurities in glycol/salt water solution.
ELF Aquitaine Production is owner of a patent directed toward the purification and regeneration of glycol solutions. This technology is said to be useful for the treatment Is of effluents from gas hydrate inhibition processes or natural gas dehydration. The method utilizes an electrically powered sandwich diaphragm system to redirect positive and negative ions in the solution into a secondary water stream. Hydrogen gas is released in the diaphragm and may constitute a problem in a plant. The mentioned technology does not appear to be commercialized.
There are a number of known deposits of gas throughout the world which have not yet been put into production, mainly due to the fear of a high salt content and the consequent problems related to disposal and purification. A satisfactory solution to this disposal problem could render more of these disposits commercially exploitable.
Therefore, there is a need for a method and a system by which it is possible to remove both water and salts in a satisfactory manner from the contaminated glycol solution.
The method should be one which may be carried out continuously. Also, some of the salts which may be present in the glycol solution, for example, barium and strontium compounds, would be considered radioactive impurities, which would require treatment in a xe2x80x9cclosedxe2x80x9d system. This need would also have to be met by a method and a system of the above mentioned type.
This task is solved with the present invention by a method for the treatment and processing of solutions of an organic fluid, water and one or more compounds of alkaline earth metals, alkali metals and metal ions, which method is characterized by comprising the following steps:
a) conducting a stream consisting of organic fluid, water and one or more compounds of alkaline earth metals, alkali metals and metal ions to a salt reduction and crystallization unit,
b) evaporating and optionally partially condensing the stream and providing thereby a stream of evaporated water and organic solvent and a stream of organic fluid and the compounds of alkaline earth metals, alkali metals and/or metal ions,
c) conducting the steam consisting substantially of organic fluid and the compounds of alkaline earth metals, alkali metals and/or metal ions to a salt reduction means,
d) nucleating crystals of the compounds of alkaline earth metals, alkali metals and/or metal ions by means of depressurization and temperature increase,
e) removing the portion of the precipitated crystals and/or particles from the organic fluid.
Additional advantageous features of the method are disclosed in the dependent claims.
The invention also relates to a means for carrying out the method, which is characterized in that it comprises a means for reduction of salts of alkaline earth metals, alkali metals and/or metal ions and a means for the separation of water and organic fluid.
Further advantageous features of the apparatus are disclosed in the associated dependent claims.
The concept is particularly well suited for continuous salt removal from large glycol systems which may be expected continuously to accumulate salts and other impurities. Consequently the present invention would be useful in polyphase transport and/or in pipeline systems which transport wet gas requiring hydrate inhibition, for the salt and other impurities would be withdrawn from the bulk glycol volume as rapidly as they enter it. This results in the control of the salt concentration while the inflow of salt is uniform and continuous, for example, as in the case of production with the gas, condensate and/or oil from the well heads. Wet gas and polyphase pipelines from the production system which have saline or contaminated water that could enter the pipeline may also derive benefit from the invention. The production system may, for example, decant the water via a three-phase separator, as the two hydrocarbon phases are permitted to enter a common pipeline. A small, but final stream of water from the three-phase separator can be expected to contaminate the fluids in the pipeline with salts, etc., because of the normally expected separation effectiveness of these types of separators. In operations where glycol is injected into the pipeline, one would expect that the salt contamination would leave the pipeline at the receival end together with the glycol.
In gas dehydration and in sweetening processes, the solventsxe2x80x94such as triethylene glycol (TEG), amines, etc.xe2x80x94are expected to absorb the same impurities as in the pipeline system, which can be the cause of foam formation in the systems, reduced efficiency of the separation process, and deposits in the regeneration system. With the aid of the present invention, this concern can already be dealt with in the planning stage, and it will be possible to control and remove the salt impurities.